Telephone system



g- L 1950 J. WICKS EI'AL 2,517,517

TELEPHONE SYSTEM Original Filed May 22, 1947 15 Sheets-Sheet 4 REPEATER CIR. 400 0392 M TANDEM TEST R47 NE 454 425M436 437 442 455 Q R435 R440 LIN SUPY RL PULSE TIMER 46 SEL. ANS SE [2E INVENTORS' John Wicks Otho D Grundsruff g- 1950 J. wlcKs EI'ALI 2,517,517

TELEPHONE SYSTEM Original. Filed May 22, 1947 15 Sheets-Sheet 5 LOOP 536 TEST REPEATER 400 RING. R560 CONT. R550 FALSE SEIZURE I N VEN TORS F g G 5 John Wicks BY Orho D. Grondsmff M 064% M AH ys 1950 J. WICKS ETAL 2,517,517

TELEPHONE SYSTEM Original Filed May 22, 1947 15 Sheets-Sheet 6 SEQUENCE R680 PAD.

REPEATER CIR. 400

INTEPPULSE TIMER R630 TONE UNIT INVENTORS PULSE John Wicks Aug. 1, 1950 J. WICKS ETAL 2,517,517

TELEPHONE SYSTEM Original Filed May 22. 1947 15 Sheets-Sheet 7- REPEATER CIR. YOO

Voice Current Repeaters 703 R739 SEND 7621: 1/ 744 FIG. 7

7| Clll PAD CONT TIME PULSE nventors BY John Wicks Otho D. Grondstuff A g- 1950 J. wlcKs El'AL 2,517,517

TELEPHONE SYSTEM Original Filed May 22, 1947 15 Sheets-Sheet 8 .lnvenrors FIG. 8 John Wicks BY Orho D Grund'stuff 1950 J. WICKS ETAL 2,517,517

TELEPHONE SYSTEM Original Filed May 22, 1947 15 Sheets-Sheet 9 REPT. CIR. 700

MON.

lnvemors FIG. 9 y John Wicks Otho D. Grandsmff Aug. 1, 1950 J. WICKS El'AL' TELEPHONE SYSTEM 15 Sheets-Sheet 12 Original Filed May 22, 1947 m. 3203 8; I 821 o e m8 gm. NE o I 1 mg N; J\ r\ F F n87 ENE J J n i i +43 08E 08E 4 l 2N; i 9.5m wcgwom o \m mnozu 02. E0 mwgmmmm INVENTORS John Wicks By Otho D. Grondstoff. M 0%. M

Aug. 1, 1950 J. WICKS ETAL TELEPHONE SYSTEM Original Fil ed May 22, 1947 15 Sheets-Sheet 15 S m mum Bwzl 8Q :75 M28 m w m V nG h m wL m m Y B 0 W m H. a d a m m @020 Tw a 55 m w wE Patented Aug. 1 195 Oak Park, 111., assignors to Automatic Electric Laboratories, Inc., Chicagoplll a corporation of Delaware Original application May 22, 1947,: Serial No. 749,758. Divided and thisapplication June. 11,

1948, Serial N0. 32,314

32; Claims. (01. nan-27 The present invention relates to telephone sys.-. tems and more particularly to tollswitching sys tems of the type disclosed in U. S. Patent No. 2,155,176, John Wicks, granted April.18,.1,939. This application 1 a division of the copending application of John Wiclts and Otho 1)., Grandstaff, Serial No. 749,758, filed May 22, 1947. i

In the toll switching system disclosedin the Wicks patent, the toll lines inter-connecting the different exchanges are terminated by repeater circuits, each of which is provided with a tone unit individual thereto of the connection and arrangement disclosed in U. S. Patent No. 2,159,081, Otho D. Grandstaff, granted May 23, 1939. Each of these tone units comprises both a transmitter and a receiver. of unmodulated audio frequency signals and fixed frequency modulated audio. frequency carrier signals that are employed for the purposes of digit control, seizure control, ring control, supervisory control and release control.

In the present toll. switching system, improved repeater circuits are provided, each of which has individually associated therewith a tone unit of the improved connection and arrangementdisclosed in the copending application of Otho D. Grandstafi, Serial No."738,l94, filed March 29, 1947.

It is a general object of the present invention to provide in a toll switching system of the type disclosed in the Wicks patent, improved circuit and operating facilities in order to accommodate thehandling with ease of heavier toll .traflic by the operators at the various toll switchboards in the different exchanges in the system.

Another object of the invention is to provide in a telephone System, an improved trunk circuit capable of routing a call either to associated switching apparatus. or to an associated operator position and selectively controlled by initial operation of the associated switching apparatus.

Another object of the invention is to provide in a telephone systenna trunk circuit accessible to an associated operator position and provided with an improved arrangement permitting. the operator at the associated operator position to change the link or cord circuit thereat. involved in an established connection without releasing the trunk circuit.

Another object ofthe invention is to provide in a telephone system, a trunk circuit accessible to an associated operator position and comprising. an improved arrangement for rendering to the associated operator position flashingsuper- VlSlOIl.

A further object of theinvention is to provide in a telephone system, a trunk circuit having a two-way. trunk accessible. to an associated opera-. tor position and comprising an improved arrangement permitting. the operator at the associated operator position to initiate outgoing calls andlto. answer incomingcalls utilizing the twoway trunk and without danger of falsely initiating. an outgoing call after the release of an in-. comingcall that, has been answered. Aiurther objectof the invention is to provide in a telephone system, a trunk circuit accessible to a plurality of. associated operator positions and comprising an improved arrangement permitting the operator at one of the associated operator positions to transfer an established connection to the operator at another of the associated operator positionsv without. danger of releasing the trunk circuit.

A still further object. of the invention is to provide in a telephone system, a trunk circuit associated with automatic switching apparatus and comprising an improved arrangement that insure the initial release of the switching apparatus before. seizure thereof for switch control purposes,

Further features of the invention pertain to the particular arrangement of the circuit elements of the system, whereby the above-outlined and additional operating features thereof are attained.

The general arrangement of the telephone system The invention, both as to its organization and method :of operation, together with further objects and advantages thereof, will bestbe understood byreference. to the following specification taken in connection with the accompanying drawings in which Figures 1 and. 2 taken together, when arranged inend-to-end relation with Fig. 1 ontthe left'and Fig. 2 on the right, constitute a schematic diagram of the area served by a telephone system embodying the present invention and illustrate the general arrangement of the apparatus incorporated in several of the exchanges and offices thereof; Figs. 3 to 14, inelusive, taken together illustrate the details of certain of the apparatus incorporated in two of the remotely disposed automatic exchanges included in the telephone system, which apparatus has incorporated therein the features of the invention as briefly outlined above; Fig. 15 illustrates the mode of combining Figs. 3 to 1.4, inclusive, .to form a unified diagram; and Fig. 16 illustrates the details of one of the tone units in Figs. 7 to 'l3, inclusive, illustrate thedetails of one of the repeater circuits incorporated in another of the automatic exchanges; and Fig. 14

illustrates a fragmentary portion or" one of the coders associated with the repeater circuit illustrated in Figs. '7 to 13, inclusive.

Referring now more particularly to Figs. 1 and 2, the telephone system there illustrated was one applied to meet the specific requirements of the system of toll lines inter-connecting a group of cities in Brazil, South America, and including.

the cities of Sao Paulo and Rio de J aneiro. Since these two cities comprise the principal cities of Brazil, each serves as aconcentration point for toll traffic; and as these two cities are within about two hundred miles of each other considerable toll traffic passes therebetween. In the general area of Sao Paulo are a number of cities, including Santos, Campinas and Jundiahy; and inthe general area of Rio de Janeiro area number of cities, including Campos. Sao Paulo comprises an automatic exchan e 33 constituting a' toll center; and a number of automatic oflices, including the automatic ofiice 385, each serving a group of subscriber substations. The automatic oflice 380 comprises office No. 8 in Sao Paulo and serves a group of subscriber substations, including the'subscriber substation Ti, as previously noted. The switching apparatus incorporated in the automatic exchange 35, constituting a toll center, as well as the switching apparatus incorporated in the automatic oifice 380, is of the well known Strowger type. Santos comprises an automatic exchange 350 serving a group of subscriber substations, including the subscriber substation T2, and automatic switching apparatus of the well known Strowger type. Campinas comprises an automatic exchange 3'13 serving a'group of subscriber substations, including the subscriber substation T3, and automatic switching apparatus of the well known Strowger type. Jundiahy comprises a manual exchange 300' and serves'a group of subscriber substations, not shown. V

Rio 'de 'Janeiro comprises an automatic exchange 53 constituting both a toll center and an office serving a group of subscriber substations, including the subscriber substation Tt. Also, the automatic exchange 53 comprises automatic switching apparatus of the well known International Standard Electric 7-A type, utilizing revertive impulse control. 'Campos comprises a manual exchange 60 and serves a group ofsubscriber substations, not shown. The automatic exchange 30 is connected to the automatic exchange 50 by a group of two-way toll lines, including the toll line 182. The toll line 732 embodies a number of voice current repeaters J53 distributed therealong; and the opposite ends thereof are terminated in repeated circuits 430 and 183, respectively disposed in the automatic exchange 36 and in the automatic exchange 55. A tone unit 6!!! in the automatic exchange 35 is individually associated with the repeater circuit 400 and a tone unit lill in the automatic exchange 50 is individually associated with-the repeater circuit 135. The automatic exchange 33 is connected to the automatic exchange 355 by'a group of two-way toll 'lines; including the toll line' 356. The toll circuits 355 and 355,

4 line 353 embodies a number of voice current repeaters 351 distributed therealong; and the opposite ends thereof are terminated in repeater respectively disposed in the automatic exchange 33 and in the automatic exchange 358. A-tone unit'358 in-the automatic exchange 38 is individually associated with the repeater circuit 355 and a tone unit 383 in the automatic exchange 361'! is individually associated withthe repeater circuit 365.

The automatic exchange 33 is connected to the automatic exchange 310 by a group of two-Way toll'lines, including the toll line 35!. The toll line 35! embodies a number of voice current repeaters 352 distributed therealong; and the opposite ends thereof are terminated in repeater circuits 350 and 315, respectively disposed in the automatic exchange 33 and in the automatic exchange 313. A tone unit 353 in the automatic exchange 30 is individually associated with the repeater circuit 350 and a tone unit 3'58 in the automatic exchange 310 is individually associated with the repeater circuit 375.-

The automatic exchange 38 is connected to the manual exchange 350 by a group of two-way toll lines, including the toll line 332. One end of the'toll line 332 terminates in a manual toll board 30! disposed in the manual exchange 335, and the other end of the toll line 392 terminates in a toll trunk circuit 3H) disposed in the automat-ic exchange 30.

The automatic exchange 30 is connected to the automatic ofiice 339 by a group of outgoing trunk lines, including the outgoing line 342, and by a group of incoming trunk lines, including the incoming trunk line 356. The outgoing trunk line 342 is accessible to the automatic switching apparatus in the automatic exchange 33 and terminates in the switch train 385 disposed in the automatic ofilce 380. One end of the incoming trunk line 306 terminates in a trunk circuit 388 in the automatic oflice 383 and is accessible via a trunk 38? to the switch train 385; and the other end of the incoming trunk line 303 terminates in a CLR trunk circuit 355 in the automatic exchange 38. In passing, it is noted that the switch train 385 in the automaticoflice 380 has access to and is accessible to the subscriber line 386 extending to the subscriber substation TI.

'The automatic exchange 53 is connected to the manual exchange Bil by way of a group of twoway toll lines, including the toll line i36i. One end-of the toll line I36] terminates at a manual toll board I320 disposed in the manual exchange 60, and the other end of the toll line I36! terminates in a toll trunk circuit I311 disposed in the automatic exchange 50.

- The automatic exchange 30 further comprises a Strowger dial toll board, the first operator position circuit of which is indicated at 3H5; which first operator position circuit 3l6 has access by way'of an associated cable 3 to a number of finder-selector links, including the finder-selector link 315. The finder-selector link 3l5 comprises a rotary finder having access to trunks 354 and 391, respectively extending to the CLR trunk cir-' cuit 305 and to the toll trunk circuit 3H3, and a Strowger selector. Individually associated with the repeater circuits 350, 400 and 355, respectively terminating the toll lines 35 l, 102 and 356 in the automatic exchange 30, are trunks 326, 390 and 33! respectively terminating in incoming toll selectors 325, 320 and 330. Also, the automatic switching apparatus in the automatic exchange 35 comprises a first group of outgoing toll selechermit tors,inc1uding the outgoing toll selector 335,-a second group of outgoing toll selectors, including the outgoing toll selector 340, anda third group of outgoing toll selectors, including the outgoing toll selector 345. The selector of each or" the finder-selector links 3l5, etc., and each of the incoming toll selectors 325, 320, 330, etc., has access to each of the outgoing toll selectors 335, 340, 345, etc., via respectively associated trunks 335, 3M, 346, etc. The outgoing toll selector 335, etc, has access to a trunk 33'l -extending tothe toll trunk circuit are and,consequently, to thetoll line 3E2 extending between the automatic exchange 3B and the manual exchange see. The outgoing toll selector 3d3,etc., has access to the outgoing trunk line 342 extending to the switch train 385 in the automatic office 383. The outgoing toll selector 345, etc., has access to trunks 395, 335 and 39'! respectively extending to the repeater circuits 350, 430 and355; and, consequently, to therespective toll lines 35l, I32 and356 extending between the manual exchange and the respective automatic exchanges iii-and 363.

The automatic exchange 3'33 further comprises a Strowger dial tollboard indicated at 33 3, and provided with automatic switching apparatus and control equipment indicated at 33?. The automatic switching apparatus 33l has access to and is accessible to the subscriber line extending to the subscriber substation T2. Also, the switching apparatus 3511s accessible to and has access to the repeater circuit 335 via connections 365; and, consequently, with respect to thetoll line 353 extending between the automatic exchange 333 l and the automatic exchange 33. The automatic exchange 3% further comprises a Strowger dial toll. board indicated at 333,. and provided with automatic switching a pparatus and control equipment indicated at 37?. The automatic switch apparatus 3l'lhas access to and is accessible to the subscriber line extending to the subscriber substation T3. Also, the switching apparatus 33'! has access to and is accessiele to the repeater circuit 315 via connections 3 15.; and, consequently, with respect to the toll line 35l extending between the automatic exchange 37.3 and the automatic exchange 3!].

The automatic, exchange 5ii-iurther comprises a manual dial toll board of the wellknown Wes er-n Electric Company No. -3 type, the first inward operator position circuit of which is indicated at I BIB, the first outward operator position circuit of which is indicated at I340, and the first transfer operator position circuit of which is indicated at i350. The first inward operator position circuit 53ft is provided with an associated group of cord circuits, oneof which is indicated at i310, and provided with an answer plug 13H and a call plug I3I2. Further, the first outward operator position circuit I340 and the first transfer operator position circuit-i351} are provided with associated groups of cord circuits, not shown, each terminatingin answer andcall plugs. Also, associated with the manual dial toll board is a display panel 1333 that is connected to the repeater circuit 303, etc, in order to display supervisory signals from the repeater circuit I93, etc. Further the repeater circuit is provided with two trunks I-3 l 3and 1333. The trunk I313 iscan incoming trunk and extends to the automatic switch train IBtt and is accessible thereto; while the trunk :I 380Jis a two-way trunk and terminates injacks J I314, 5134i and Ji:35I-, respectively dis-- posed at the first inward opei ator position circuit lslaat the first outward operator '-position circuit I 340, and at the first transfer operator post tion circuit I35ll,' disposed at the manual'dial toll board in the automatic exchange 50. Also, the automatic switch train 2333 has access to and is accessible to the subscriber line I30I. extend.- ing to the subscriber substation T4. Further,

theautomatic switch train I 330 has access to a groupoi outgoing trunks, including the outgoing trunk I332, and isaccessible to a group of incoming trunks, including the incoming trunk I363. Theoutgoing trunk I362 is terminated in a CLR trunk circuit I332 and the incoming trunk 8353 terminatesin a toll trunk circuit I303. The toll trunk circuitI3fl3, etc., the CLR trunk circuit 5382, etc., and the toll trunk circuit I3Il, etc., respectively terminate in jacks J i338, J i309 and J l3l3 disposed at the first inward operator position circuit I3I6 and .multipled to similar jacks, not shown, disposed. at the first outward operator position circuit I343 and at the first transfer operator position circuit I358.

Further, a group of toll register senders, including the toll register sender NM, and a group of local register senders, including the local register sender I332, are commonly associated with the automatic switch train I333 in the automatic exchange 53. Further, in the automatic exchange 58 there is disposed a preselector I433 having access to a group of coders, including the coder Milt, andL to the repeater circuit lilo, etc. Specifically, the preselector I403 has access to the coder l lii l via a cable i420 and access to the repeater circuit icllvia a cable I4 I ii. Also, the automatic exchange 53 comprises a group of finder circuits, including the finder circuit Mill, and an associated group of register senders, including the register sender M32, as well as an associated allotter I465. The allotter M05 is commonly associated with repeater circuit 'Illll, etc., and with the finder circuit Hill I, etc.; while the finder circuit Mal, etc, has access to the repeater circult "lllil, etc., via a cable I090, etc. Finally, the finder circuit Mel is individually associated with the register sender I402.

The apparatus ncorporated in the telephone system In the automatic office 383. the switch train 385 is of the well known Strowger type, as previously noted, while the trunk circuit 383 may be of any suitable type. I I

Inthe automatic exchange 360 thetoll switchboard 369 is of the well known Strow-ger dial type, as previously noted, and may be identical to the Strowger dial toll board provided in the automatic exchange 39. Similarly, the automatic switch and control equipment '36? disposed in the automatic exchange 360 is of the well known Strowger type, 'as previously noted, and may be identical to the corresponding automatic switch and control equipment disposed in the automatic exchange 3!]. Further, therepeater circuit 335 disposed in the automatic exchange 389 is identical to the repeater circuit 355 disposed in the automatic exchange 31]; which, in turn, is identical to the repeater circuit 490 described in detail hereinafter. Finally, the tone unit 338 disposed in the automatic exchange 363 is identical to the tone unit 358 disposed in the automatic exchange 30; which, in turn, is identical to the tone unit I600 described in detail hereinafter.

In the automatic exchange 310 the toll switchboard 3'l9is of the well known Strowger dial type, as previously noted, and may be identicalto the Strowger dial toll board provided in the automatic exchange 30. Similarly, the automatic switch and control equipment 311 disposed in the automatic exchange 310 is of the well known Strowger type, as previously noted, and may be identical to the corresponding automatic switch and control equipment disposed in the automatic exchange 30. Further, the repeater circuit 315 disposed in the automatic exchange 310 is identical to the repeater circuit 350 disposed in the automatic exchange 30; which, in turn, is identical to the repeater circuit 400 described in detail hereinafter. Finally, the tone unit 318 disposed in the automatic exchange 310 is identical to the tone unit 353 disposed in the automatic exchange 30; which, in turn, is identical to the tone unit I600 described in detail hereinafter.

In the automatic exchange 30, preferably the CLR trunk circuits 305, etc., are identical and are of the form illustrated in Fig. 3 of the previously mentioned Wicks patent; and the toll trunk circuits 3"], etc., are identical and are of the form illustrated in Fig. 4 of the previously mentioned Wicks patent. Also, in the automatic exchange 30 the various operator position circuits 3I5, etc., at the Strowger dial toll board therein are identical and are of the form illustrated in Fig. 5 of the previously mentioned Wicks patent. Similarly, the finder-selector links 3I5, etc., are identical and are preferably of the form illustrated in Figs. 5 and 6 of the previously mentioned Wicks patent. Preferably, the incoming toll selectors 325, 320, 330, etc., are identical and are of the form illustrated in Fig. 6 of the previously mentioned Wicks patent; the outgoing toll selectors 345, etc., are identical and are of the form illustrated in Fig. 8 of the previously mentioned Wicks patent; the outgoing toll selectors 340, etc., are identical and are of the form illustrated in Fig. 9 of the previously mentioned Wicks patent; and the outgoing toll selectors 335, etc., are identical and are of the form illustrated in Fig. 10 of the previously mentioned Wicks patent. As previously noted, the repeater circuits 350, 355, etc., are identical to the repeater circuit 400 illustrated in Figs. 4 to 6, inclusive, and described in detail hereinafter; while the tone units 353, 60I, 358, etc., are identical to the tone unit I600 illustrated in Fig. 16 and described in detail hereinafter.

In the automatic exchange 50 the automatic switch train I300 is of the International Standard Electric 7-A type, as previously noted; the manual dial toll board is of the well known Western Electric Company No. 3 type as previously noted, cord circuit I3I0 and the position circuit I3I6 being of the forms respectively in Figs. 20 and 21 of the previously mentioned Wicks patent. Preferably, the toll register senders I39l, etc., and the local register senders I362, etc., are of the usual types; and the toll trunk circuits I303, etc., the CLR trunk circuits I302, etc., and the toll trunk circuits I3I1, etc., may be of any suitable types. Further, the tone units 10I, etc. are identical to the tone unit I600, illustrated in Fig. 16 and described in detail hereinafter. Preferably, the repeater circuits disposed in the automatic exchange are identical to the repeater circuit 100, illustrated in Figs. 7 to 13, inclusive, and described in detail hereinafter. Preferably, the preselector I403 is of the form illustrated in Fig. 15 of the previously mentioned Wicks patent; and the coders I404, etc., are identical and, as indicated in Fig. 14, are substantially of the form il ustrated in Fig. 1.6 (parts 1, 2. and. 3) of the previously mentioned Wicks patent. Preferably, the finder circuits I40I, etc., are identical and are of the form illustrated in Fig. 17 of the previously mentioned Wicks patent; the register senders I402, etc., are identical and are of the form illustrated in Fig. 18 (parts 1 and 2) of the previously mentioned Wicks patent; and the allotter I405 is of the form illustrated in Fig. 17 of the previously mentioned Wicks patent.

The repeater circuit 400 disposed in the automatic exchange 30 comprises, as shown in Figs. 4 to 6, inclusive, a relay group including a series relay R4I0, a release relay R420, a line relay R430, two release pulse timer relays R435 and R440, a selector seize relay R450, an incoming release relay R460, a tandem test relay R410, a line supervisory relay R480, an answer supervisory relay R490, a re-ring relay R400, an answer supervisory relay R5I0, a supervisory control relay R520, a supervisory relay R530, a ring relay R535, three ring control relays R540, R550 and R560, a switch-over relay R510, a dial-in relay R580, a control relay R590, a loop test relay R500, a control relay R505, a supervisory relay R6I0, a signal-in relay R620, an interpulse timer relay R630, a switch-through relay R640, a guard relay R650, a supervisory pulse relay R660, a prepare relay R610, a send relay R615, a sequence relay R680, a pad control relay R690 and a pad cutout relay R600, as well as a toll pad 695.

The repeater circuit disposed in the automatic exchange 50 comprises, as shown in Figs. 7 to 13, inclusive, a relay group including a pad cutout relay R1l0, a pad control relay R120, a send relay R130, a switch-through relay R140, a prepare relay R150, a cutoff relay R160, a time pulse relay R110, an auxiliary relay R180, a rering relay R8l0, a control relay R820, an end-ofdial relay R830, a supervisory relay R840, a supervisory start relay R850, a loop test relay R860, a

signal-in relay R810, a dial-in relay R880, a selector seize relay R890, a switch-over relay R9I0, an incoming release relay R920, a control relay R930, a coder seizing relay R940, a control relay R950, a ring relay R960, a sequence relay R910, a finder stop relay R980, a monitor relay R990, a release pulse timer relay RIOI0, an answer relay RI020, a release pulse timer relay RI030, a supervisory relay RI040, a series relay RI050, a line relay RI060, a sleeve relay RI010, a start relay Rl080, two transfer relays RIIIO and RII20, a coder supervisory relay RII30, a register sender supervisory relay RI I40, a control relay RI I50, a flash relay RI I60, two re-ring relays RI I10 and RI2I0, a sleeve relay RI220, a cutoff relay RI230, a signal relay RI240, a position transfer relay Rl250 and two switch release relays RI260 and RI210, as well as a toll pad 195.

As previously noted, the coder I404 disposed in the automatic exchange 50, shown in skeleton diagram in Fig. 14, is substantially of the form illustrated in Fig. 16 (parts 1, 2 and 3) of the previously mentioned Wicks patent; however, the coder I404 is slightly modified in that it includes an additional relay, the transfer control relay Rl430, which is utilized in carrying out an additional feature incorporated in the coder I404 and explained more fully hereinafter. Otherwise, the coder is of the form mentioned and comprises the six identical digit storage devices DSI to D36, inclusive; the digit storage device DSI being illustrated as including the four individual relays A, B, C and D.

The tone unit I600 is of the type disclosed in 9. the previously mentioned copendingapplication of Otho D. Grandstaff, and essentially comprises, as illustrated'in Fig. .16, a receiving transformer TRI, a sending and coupling transformer TRZ, a monitoring transformer TRB, a two-way amplifier Tl in the form of a vacuum tube of the well known pentode 6C6 type, a difierehtial detector T2 in the form of a vacuum tube of the well known dual diode 6H6 type, an impulse amplifier T3 in the form of a vacuum tube of the' well known pentode 606 type and a signal amplifier T4 in the form of a vacuum tubeofthe well known pentode 43 type, as well as a relay group including a modulation cutoff relay RH-329, a receiving switch relay RIMI], a pulse cut-in relay RI 650, an alarm relay RI655, an impulse-in relay R1660, and a signal-in relay R [610; together with a plate impedance I628 associated withthe tube Tl, a plate impedance i634 associated with the tube T3, and a plate impedance i639 associated with the tube T4. Furthenthe tone unit I690 comprises circuitelements described in detail hereinafter and inter-connecting the elements mentioned, as well' as a group of conductors extending to the associated repeater circuit, and including a sending conductor Cltlll, a sending conductor Cl6l12, a monitoring conductor Cl6ll'3, a monitoring conductor CIGM, a receiving conductor (N685, a receiving conductor Clfiutfa receiving switch conductor Clfiill, a modulation cutoff conductor (H638, a signal-in conductor Cl6ll9, a dial-in conductor ClSIG, and a pulse cut-in conductor clfill; together with a'60 cycle conductor C|6|2 extending to an associatedGO cycle generator, not shown,

and a 1000 cycle conductor Cl6l3 extending to an associated 1000 cycle generatonnot shown.

The operation of the tone unit 1600 In the tone unit I690, thetiibes Ti and T3 are each provided with a cathode heater having a nominal rating of 6.3 volts, at which voltage 0.3 ampere is drawn; the tube T3 is provided Witha two-seetion cathode heater having a nominal rating of 6.3 volts, at which voltage 0.3 ampere is drawn; and the tube T-t is provided with a cathode heater having a nominal rating 01"25 volts. at which voltage 0.3 ampere'isdrawn. The cathode heaters of the tubes Tl, T2, T3,and T4 are connected in series circuit with'a 4 ohm resistor RI and the winding of the alarmrelay RI555 across a 4.8 volt exchange battery, the positivepole of which is grounded. More p'articulailmthe circuit mentioned extends fromground by way of the contactsit'lfl of a normally OperatedkeyKZ, the cathode heater of the tube T2, a conductor Clfild, the cathode heaterof the tubeTZi, a conductor CHEM, the cathode heater or the tube Ti, a conductor C1615, the resistor Ri', a conductor came, the cathode heater oi the tube Ta, and the Windingof the alarm relay R l 655 to thenegative pole of the 48 volt exchangebattery. Accordingly, the series circuitabove traced is normally completed, whereby the cathode heaters heat the associated cathodesof the tubesmentioned"rendering them electron emissiv'e and effectingop eration of the alarm relay EH65. Of course, it will he understood that in the evcnt'any one of the tubes mentioned is removed from its socket or develops a burned out or br'oken cath'ode heater, the above traced series circ'iiit isinterrupted, causing the alarm relay R1555 to restore in order to complete, at the contacts [656, a circuit including the contacts Hits of the key K2for illuminating a cathode'heaters alarm lamp 1,111 657,

thereby to indicate the failure mentioned. Moreover; the potential drop along theabove-traced series cathode heater circuit is employed at various points forgrid biasing purposesexplained more fully hereinafter; and, in passing, it is noted that a negative potential of 6.3 volts appears upon the conductor (H613, a negative potential of 18.9 volts appears upon the conductor Cifil'z', and a negative potential of 20.1 volts appears upon the conductor ClBlii. i a i The tuhe'Tl is employed as a'two-way amplifier, being utilized as an outgoing amplifier and modulator inthe transmission of signals to the asso ciated repeater circuit. The screen grid ofthe tubeTl is supplied with positive potential from a 130 volt source'oi plate current, the negative pole of which is grounded, through an associated 50,0il0bhm resistor I629 and is by-passed to the cathode of the tubeTl; by will mf. condenser 153i! connected therebetween; and the suppressor grid of the tube Tl, employed as a modulation grid, is connected either to ground potential or to the cycleconductor CEGIE, depending upon the operated or restored condition ofthe modulation cutofi relay RIGZB and the receiving switch relay Rlt lil as explained more fully'hereinafter.

For the reception of signals, the primary windingof the receiving transformer TR! is connected to the receiving conductorCifiilE and. to the receiving conductor Cliiflfi through the two illustrated 0.1 mi: condensers; :the secondary winding oi the receiving transformer TB! is normally connected by wayof the contacts I64! of the normallyhperated receiving switch relay Rit lil tothe control grid of the tube Tl in order to controlthe potential applied thereto; a negative potentialof 20.1 voltsbeing normally applied to the controlg'rid of the tube Tl from the conductor CI616 by wayof a 0.15megohm resistor R2, the secondary windingoi the receiving transformer TR-I and the contacts IBMof the receiving switch relay R1640." The cathode of the tube TI is normally connected by way of the contacts llidtpf the normally operated receiving switch relay RI are to theconductor Clfilii having negative 18.9 voltpotential applied thereto. Accordingly, the control grid of the tube Tl has a negative biasing potential of 1.2 volts applied thereto with? respect to the cathode by virtue of the 1.2 volt drop across the resistor RI. i With the receiving switch relay RIBM) in it normal operated position, the suppressor grid of the tube TI is connected by way of the contacts I545 thereofto ground potential, as previously noted andthe secondary winding oi the sending and coupling transIOrmer'I-RZ is connected, at the contacts I642 and lfiM of the operated receivingswitch relayRl 640, across the leads CI 525 and Clfiiit. The primary windingof the monitoring transformer- TRS is connected by Way of the two 5000 ohm resistors i623 and i624 across the leads C1625 and CIBZB; while the secondary winding oi the monitoring transformer IRS is connected directly to the monitoring conductor-C1583 and the monitoring conductor CIEM extending to the associated repeater circuit. Thus, audible signals amplified by the tube TI and impressed through the sending and coupling transformer TRZ across the leads Clii25 and (H525 are supplied through the monitoring transformer TRB to the monitoring andmonitoring conductors Cllilifl and CH'iM extending to the associated repeater circuit," for a purpose, more wily-explained hereinafter. Further, the plate 

